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Finding force of friction

  1. Nov 23, 2010 #1
    1. The problem statement, all variables and given/known data
    I have a lab project in physics and I'm having a bit of trouble. We measured the Applied force of a moving block of wood using a spring scale. The first trials were don't with just the table , knowing the mass and applied force I was able to find the friction force; simply it was the same as F app. My problem now is that we did other trials on sandpaper, and I have the Applied force and mass and I don't know how to find the mass. We are suppose to find the Co efficent of friction, but not through math ; we have to use a graph where : y = mx aka Ff=U*Fn

    Which isn't too bad, I just can't figure out how to get Force friction with only the mass and applied force when it is using the sand paper.

    Mass = 0.27952 kg Force applied = 2.0N
    Fn = Fg
    mg= 0.27952(9.8)= 2.739296 N

    Help please

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 23, 2010 #2


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    I am assuming that the block or mass is moving slowly or at constant speed when the force is applied...in the 2nd example, you know the applied force so you then know the kinetic friction force, and since you have calculated the normal force, plot that point on the graph with Ffriction on the y axis and the normal force on the x axis. Then try another mass, and do the same, moving it slowly; and yet another....your graph should approximate a straight line, the slope of which is____?
  4. Nov 23, 2010 #3
    Can't find friction, yes I have the Force normal , but no coefficient. Even if I know the applied force I don't know how that helps me find friction example

    Fapp-Ffrict = MA

    I only know F app and mass

    Ffrict = U * Fn

    Don't see how you solve unless Fapp in the 2nd problem also equals Ffrict.
  5. Nov 23, 2010 #4


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    That's right, and that is why i mentioned that in your experiment, you move the object slowly so it doesn't accelerate (a =0). Then Fapp-Ffrict = 0, or Fapp = ffrict. If you use too much of a force and the object accelerates, now you need a means of determining the acceleration. Instead, move it s-l-o-w-l-y, without accelerating. Per newton 1, an object at rest or moving in a straight line at constant speed, will remain at rest or moving in a straight line at constant speed, unless the net force acting on it is greater than 0. Keep it at constant speed, and F_net = 0.
  6. Nov 23, 2010 #5
    Oh true, didn't even think about it. Ha.

    Thank you , appreciate it.
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